1. Field of the Invention
Embodiments of the present invention relate to a unit pixel of a color image sensor and a photo detector thereof.
2. Description of the Related Art
An image sensor is a sensor that transforms an optical signal to an electrical image signal. When light is irradiated to a light-absorbing part inside a unit pixel of an image sensor chip, the image sensor detects the light incident at each unit pixel and the amount of the light and transforms an optical signal to an electrical signal and then transfers the electrical signal to analog and digital circuits for forming an image.
The conventional image sensors can be classified into CCD (Charge Coupled Device) types and CMOS (Complementary Metal Oxide Semiconductor) types, based on their structures and operation principles. The CMOS type image sensors are commonly referred to as CIS (CMOS Image Sensor).
In the CCD type image sensor, groups of signal electrons generated at the pixels by the light are transmitted to an output unit by a pulse applied to a gate, transformed to voltages of the output unit, and sent out one by one.
In the CMOS type image sensor, the signal electrons and holes that are generated at the pixels by the light are transformed to voltages inside the pixels. These voltages are connected to a signal processor, including a row decoder and a column decoder, and sent out of the pixels by a switching operation according to a clock frequency.
One electron-hole pair (EHP) is generated for one photon that is incident at a unit pixel light-absorbing part of an image sensor, and the generated electrons and holes are accumulated in a photo diode, which is a light-absorbing part.
The maximum accumulation electrostatic capacity of a photo diode is proportional to the area of photo detection of the photo diode. Particularly, in the case of CMOS type image sensor, the area in which the accompanying transistors are arranged is larger than that of the CCD type image sensor, and thus increasing the area of the light-absorbing part is physically limited. Moreover, the photo diode, which is commonly used as the light-absorbing part of an image sensor, has relatively small electrostatic capacity and thus is easily saturated, and it is difficult to segment the signals generated by the light-absorbing part.
Therefore, the unit pixels of the CMOS image sensor require a relatively long photoelectric charge accumulation time in order to generate a minimum electric charge for signal processing through the limited photo detection area. Accordingly, it is not easy to manufacture a high-density/high-speed frame image sensor by using the unit pixels having this kind of light-absorbing part.
The band gap of a silicon semiconductor is 1.12 eV, and a photo detector made of a silicon semiconductor can detect light energy in wavelengths of 350 nm to 1150 nm. Here, since the light has different inherent energy per wavelength and has different depth of penetration when the light penetrates silicon, which is solid, the photoelectric efficiency for each wavelength is also different at the photo detector. In order to detect the wavelengths of visible rays (400-700 nm), the image sensor forms an interface of P-N junction so that a green ray, which commonly has energy in the wavelength of 550 nm, can be better detected. Therefore, in the image sensor having this structure, photoelectric efficiencies for a short wavelength, such as blue color, and a long wavelength of a near infrared ray are deteriorated, or the optical signal is transformed to a noise.
Prior arts related to an image sensor and a unit pixel of an image sensor include U.S. Pat. No. 5,965,875 (“COLOR SEPARATION IN AN ACTIVE PIXEL CELL IMAGING ARRAY USING A TRIPLE-WELL STRUCTURE”) and U.S. Pat. No. 7,623,165 B2 (“VERTICAL TRI-COLOR SENSOR”).
U.S. Pat. No. 5,965,875 discloses color separation in an active pixel MOS imaging array using the differences in absorption length in silicon of light of different wavelengths. For this, the array of U.S. Pat. No. 5,965,875 uses a triple-well structure to ensure that each pixel sensor measures each of the three primary colors (RGB) in the same location.
However, U.S. Pat. No. 5,965,875 requires a certain distance of separation between adjacent unit pixels due to the property of the triple-well structure, inevitably increasing the size of unit pixels. Moreover, since three P-N junction photo diodes are generated through the triple-well structure, the problem of low sensitivity, which is a limitation the photo diode method has, cannot be solved. Furthermore, due to the property of the triple-well structure, the pixel sizes become smaller from the red pixels to the green pixels and then to the blue pixels. Therefore, some of the blue light incident at a color array sensor is incident outside the blue pixels, and some of the green light is incident outside the green pixels, thereby lowering the sensitivity of the color array sensor for the blue light and green light and failing to completely render the blue and green colors.
U.S. Pat. No. 7,623,165 relates to a vertical tri-color sensor having vertically stacked blue, green and red pixels, and discloses that the vertical tri-color sensor detects blue, green and red components of incident light by converting the blue, green and red components to surface plasmons.
However, in U.S. Pat. No. 7,623,165, since green light and red light are reflected by a plurality of vertically stacked metal layers because a particular wavelength of incident light is selectively transmitted using the surface plasmon phenomenon, the sensitivity of color array sensors for the green and red lights is deteriorated, and the actual green and red colors cannot be realized. Moreover, since a wave form of metal layer having a particular period and thickness needs to be generated between dielectric layers in order to realize the surface plasmon phenomenon, the manufacturing processes are complicated, and the manufacturing costs are increased. Furthermore, a blue stack, a green stack and a red stack need to be separated and manufactured and then stacked together in order to manufacture the color array sensors, the manufacturing processes become complicated, and the defect rates are increased.